iTunes playlist oddness

I was messing about transferring some playlists to micro SD card for
my phone using the excellent SyncTunes last night when I noticed some
weirdness in iTunes.
I looked at the contents of a playlist, noticed it didn't have a
couple of albums I wanted, and so I added them.  I then got the these-
are-already-there-skip-cancel-ok dialogue box.  Hmm.  Weird.  I
noticed that if I restart iTunes and look at the playlist then I can
see everything in the list correctly.  However if I flick between
different playlists they quite often show as incomplete - i.e. there
are items in them that don't display in the list, although they *are*
there as evinced by the results when you copy these items into the
playlist again.

Has anyone seen this?  I updated iTunes to latest via Software Update
a while back, which may have something to do with it.

Any suggestions for a fix?